Tunnel kiln



May 10, 1932. E. BJFORSE ET AL TUNNEL KILN Filed May 9. 1928 4 Sheets-Sheet INVENTOR May 10, 1932. E. B. FORSE ET AL TUNNEL KILN Filed May 9. 1928 4 Sheets-Sheet 2 INVENTOR s -May 10, 1932. E. B. FORSE ET AL 1,853,008

TUNNEL KILN Filed May 9. 1928 4 Sheets-Sheet 4 Patented May 10, 1932' UNITED STATES;

\PATEQNTI OFFICE EDWIN B. FORSE AND CHARLES E. GEIGER, OF METUCHEN, NEW JERSEY, ASSIGNOBS TO THE CARBORUNDUM COMPANY, OF NIAGARA FALLS, NEW YORK, A. CORPORATION OF PENNSYLVANIA.

TUNNEL KILN Application filed May 9, 1928. Serial No. 276,222.

This invention consists in providing tunnel kilns with combustion chambers of the radiating combustion type so that an unusually large percentage of the heat is uti- 5 lized in radiant form in addition'to which it is possible to utilize the products of combustion for contact heating. Further improvement lies in a means and method for controlling the rate of cooling of ware after it 10 has been subjected to maximum temperature conditions in the kiln, and utilizing the heat absorbed from the cooling ware in preheating air for use in the combustion units of the riln. A further object'of the invention is to provide for controlling the atmosphere in various portions of the kiln in such manner that any section may be made reducing, neutral, or oxidizing, as desired.

Our invention and the method of operating the same may be readily understood by reference to the accompanying drawings which illustrate an embodiment of our invention, and in which Figure 1 is a plan View partly in section and partly in outline, of a kiln constructed in accordance with our invention; I

Figure 2 is a longitudinal vertical section along the center of the kiln shown in Figure 1;

Figure 3 is a transverse vertical section on a larger scale in the plane of line IIIIII of Figure 1;

Figure 9 is a longitudinal vertical section through still another form of radiating coinbustion chamber or unit;

Figure 10 is a transverse sectionon a larger scale in the plane of line XX of Figure 1; Figure 11 is a similar view .in the plane of line XI.-XI of Figure 1;. and

Figure 12 is a transverse vertical section Figure 4 is a similar section in the plane.

on a larger scale in the plane'of line XII- XII of Figure 1. p

In Figures 1 and 2 the scale of the drawings as compared with a full sized kiln is so small as to render thesefigures merely more or less diagrammatic. In Figures 3 to 12, inclus ve, the sections are on a relatively larger scale, and the-preferred details of the wall construction and the arrangement of the various refractories is more clearly apparent. Furthermore, in Figure 1' the portion of the kiln below the level of the radiati ng combustion chambers has not been shown,

for the reason that this trackway is well un- .into a number of zones, according to the type of ware which is to be treated in the kiln. For certain types of ceramic ware the kiln preferably has three zones of rogressively increasing temperature, hereina ed zone a, zone 6, zone a, and followed by a cooling zone. In Figures 1 and 2 the ware travels through the kiln on carswhich move along. tracks 2 in the bottom of the tunnel 3. The direction of travel of these cars is r designatopposite to the directionof the arrows in Figures 1 and 2, the arrows indicating the direction of gas and air flowing through certain parts of the kiln, as hereinafter described. The first zone a may be considered the first part of the kiln which the ware enters and extends from point'A to point B inFigures 1 and 2. The nextzone 5 extends from point B to point C. The third heating zone 0 extends from point C to. point D, and

from D to E is the cooling zone. Beyond the cooling zone E is a further cooling zone with which metal wallsmay be used.

In the second zone I), at each side of the kiln and preferably directly opposed to each other, are heating unitswhich are preferably of the well known radiatin combustion type. These combustion units, Ilesignated 4, are formed of a silicon carbide refractory and may be of any of the forms shown in Figures portion of the length of the kiln.

55 of the zone 0.

7, 8 and 9, as will be hereinafter more fully described.

In the last zone where ordinarily the final burning or firing of the ware takes place, relali tively high temperatures must be secured.

Located in this zone ateach side of the tunnel 3 and supported on ledges 5, as best shown in Figure 4, are one or more radiating combustion chambers 6 of a construction similar 30 to the radiating combustion chamber 4, but

capable of burning considerably more fuel.

In Figures 1 and 2 we have shown the kiln as being provided with two of the combustion units 6, on each size of the tunnel, the com- Yw bustion units being turned in opposite directions. Each combustion unit 6 is provided with a fuel port 7.

The cooling 'zone of the kiln is of such construction that the rate of cooling can be regulated and controlled to a considerable extent, and the temperature of the ware is re duced in this zone as rapidly as possible compatible with safe operation. In order to provide for cooling at a definite rate the walls of the cooling chamber are made of a refractory whose heat conductivity is in exoess of .006/cal/cm /sec/C. and of a thickness preferably not exceeding nine inches. Behind these walls, which are designated 8,

- and within an outer containing wall 9 are air circulating passages 10 throu h which air may be circulated in any suita ledirection. As shown in Figure 11 the air circulates from a point near the bottom of the kiln up the passages 10, through the openings 11 arranged along the top of the outer shell into a collector 12 that extends along the top of the cooling zone of the kiln. The collector 12 communicates through a part of passage 13 4E with a longitudinally extending conduit. 14,

the conduit 14; extending along the greater As shown in Figure 1 the conduit 14 is provided with lateral branches at 15, 16 and 17. The lateral branch 15 communicates with a vertical passage 18 shown in detail in Figure 5, and each of these vertical passages communicates at its lower end with the burner end of-one of the combustion units 6. A damper plate 19' is provided for controlling the flow of air Due to the high thermal conductivity of the walls 8 of the cooling chamber, heat radiating from the ware is absorbed by the walls 8 and conducted throughthe walls. 'Air flowing through the passages 10 absorbs the heat, and as the cooling effect is roughly proportional to the velocityof the air, so that by regulatregulated, and the ware can be cooled as rapidly as is consistent with sound practice.

Silicon carbide is particularly applicable as a material for the walls 8 due to its high thermal conductivity and its property of readily absorbing heat radiated and conducted thereto and transferring to it the air circulating in back of the wall. Between veloci-' ties of ten and sixty feet per second, the rate of cooling varies almost directly with the velocity of the air. For instance, a silicon carbide brick of high density has been found to possess a thermal conductivity approximately nine times that of fire clay. At the same time it is much stronger than fire clay. By reason of this fact a silicon carbide wall, to carry the same load, can be thinner than a fire clay wall. If made of a thickness equal to the thickness of the usual fire clay wall, it will not only be stronger, but it will conduct heat therethrough approximately nine times as fast. Assuming a thermal conductivity of nine, compared with fire clay, walls of equal thickness formed of different refractories would possess the following properties:

Fused Magnesic v A120 site Clay 1 Thickness Same Same Qame I Same Thermal conductivity 9 2.5 2.5 I 1 Transverse str. at 1350 0 1500 200 140 200 Crushing str. at 1350" C. 1000 200 50 l 50 Resistance to fused ash" 10 1 0. 4 1 Resistance to abrasion 3 2 01 1 Resistance to spalling 1 $5 0. 3 1 Fusion point 2240 C. 1900 C. 1900 C -1750 C (Decoml poses) i According to this, walls of the different refractories having equal thermal conductivities would necessarily'have different thickness. For walls having equal thermal conductivities the properties are as follows:

- Fused Magne- SIC A120 I site Clay Thickness 9 2. 5 2. 5 1 Thermal conductivity Same Same Same Same 'Transverse str. at 1350" C 121. 500 125 875 200 Crushing str. at 1350 C 9, 000 '500 150 50 Resistance to fused ash" 90 2. 5 1 1 Resistance to abrasion-.. 27 5 0. 03 1 Resistance to spalling 9 1. 0. Fusion point 2240 C. 1900" C 1900 C. 1750 C.

(Decom poses) high thermal conductivity the rate of. cooling of the ware can be definitely controlled by regulating volume and velocity of air circulated through the cooling passages backof the wall 8. A fire clay wall having a suflicient mechanical strength would be such a poor conductor of heat that any control of the rate of cooling would not be possible.

Silicon carbide is also preferable to metal because it can be used at much higher temperatures than metal, so that much h1gher temperatures and more gradual cooling can obtain in the cooling zone.

It is because of these same properties of silicon carbide that the radiating combustion units are also preferably made of this refractory.

, The air which is preheated in passing through the walls of the cooling chamber is utilized in supporting combustion in the various radiating combustion units 4 and 6, thereby effectively increasing the operating temperatures of these units and the efliciency of the kiln.

The products of combustion from the radiating combustion units 6 and 4 are preferably discharged directly into the tunnel 3 and move along the kiln in a direction counter to the movement of the ware through the tunnel. The invention in its broader aspects, is not confined, however, to a structure wherein the products of combustion are discharged into the tunnel, but also applies to cases wherein the products of combustion are separated at all times from the tunnel by a heat transmitting partition, as in the ordinary mufile kiln.

The radiating combustion units 4 and 6 may be of any suitable size and construction and may, for instance, be of the type disclosed in Hawke Patent No. 1,594,834, of August 3, 1926, wherein there is a double passage for the flame, or may have only a single pass. Various forms of combustion units are disclosed in Figures 7 to 9, inclusive. In the form of combustion unit shown in Figure 7, there is but a single passage or chamber 22 having projections23 thereon with discharge ports or openings 24 therein communicating with the interior of the chamber. In the construction shown in Figure 8 the combustion unit has two passages 25 and 26. The passages are connected at one end through a cross passage 27. The two passages 25 and 26 are maintained in spaced relation by spacing blocks 28, and this spacing of the passes 25 and 26 provides increased radiating surface for the passages, as described in said Hawke patent above referred to. The passage 26 has ports 29 therein. The

1 unit shown in Figure 9 is substantially simi lar to the one shown in Figure 8 except that the passages, instead of being located side by side, are disposed one above the other.

In Figure 9 there is a closed upper passage 31 with a burner 32 at one end. The other end of the passage 31 connects through a vertical passage 32* with a lower passageway 33 in which are outlet ports 34. The upper and lower passages 31 and 33 are held apart by spacing blocks 35. In order to be efi'ective the radiating combustion units must be of a move the mechanically held water from the ware. The atmospheric composition of the gases in this portion of the tunnel is not important, though it is important that the moisture content be fairly low and the circulation of gases be good. In some instances this zone might for convenience be separate from the remaining portion of the kiln, although we have shown it as being a part of the main kiln structure.

In the second zone b the ware is heated to a much higher temperature thanit is in the first zone. For instance, in the burning of some wares the heat may be such that the ware will be heated to a bright red. This zone is a relatively long one and the material preferably remains in this zone for a considerable period of time.

In the last heating zone a the temperature is brought up to the highest point to which it'is desirable to fire the ware, and the atmosphere is maintained which is most desirable for the ware in question. The ware, while it is in this zone, is immediately alongside of the combustion chambers, and from the combustion chambers the gases flow into the kiln and along the kiln, as previously described, although if desired the gases can be removed at any point by the provision of fines, as is well understood in the art. The atmosphere in this zone may be oxidizing, reducing or neutral according to the character of the ware being fired.

A radiating combustion chamber lends itself effectively to the control of the kiln atmosphere. For instance, the fuel may be introduced into the combustion unit in such a way that complete combustion will not be effected entirely within the chamber, but the gases or flame coming from the chamber will contain considerable unburned fuel whose combustion will be completed in the kiln. In such a case a reducing atmosphere is maintained in the last heating zone. On the other hand, combustion of the fuel may be made complete within the combustion unit, with little excess air so that the gases are substantially neutral. By burning the fuel completely in the combustion units in conjunction with an excess amount of air which is highly heated in the combustion units, the atmosphere in this zone of the kiln may be highly oxidizing. After the ware passes In order that the cooling of the ware may be properly controlled the passages are preferably arranged in roups along the walls of the cooling zone. uch groups are indicated at m, 3/ and a in Figure 1, and any suitable means, such as dampers as diagrammatically indicated at 10 in Fig. 11 may be employed for controlling the amount of air circulated through the passage.

' As shown in Figures 3 and 4, the radiating combustion units being built in ledges along the inside of the tunnel, have an inner face and a top situated to radiate heat direct ly into the kiln. By using radiating combustion units formed of a silicon carbide refractory the percentage of heat which is radiated far exceeds the percentage of heat radiated from units of other refractories. Under proper conditions approximately of the heat may be radiated.

It has been found {in operating continuous direct fired tunnel kilns at high temperatures that considerable expense is entailed in maintaining the under parts of the cars, such as the sand seal plates, wheel bearings, 'ournal boxes and the like, sufiiciently cool. ccording to our invention there is provided a ventilating system which cools the underparts of the cars by natural circulation, as will be clearly apparent from an inspection of Figure 4. In Figure 4 we have shown the main bodyof the kiln as being carried on structural iron channels 40 and cast iron plates 41, providing transversely extending ventilating ports or passages 42 in the lower part of the structure. At 43 are sand troughs. One of the cars is designated 44: and has plates 45 on the sides thereof projecting down into the sand seal troughs 43.

l/Vith this arrangementcold air circulates along the bottom of the passages 42, as shown by the arrows, where it fiows against the rails 2 and against the bottom parts of the cars. It cools the rails and the bottom plates of the car, and escapes out the tops of the passages 42. The natural circulation of air is thus maintained where ever the cross ports 42 are provided, and these can be located at any point along the kiln where excessively high temperatures are employed.

Any material leakage of air from the under sides of the cars into the ware chamber of the kiln is prevented by the sealing afforded with the deep embedding of the sand seal plates 45 in the sand carried in troughs 43. This is particularly the case in oil fired kilns because the ware chamber is not maintained at any appreciable negative pressure or draft. Moreover with the type of combustion chambers employed as herein described, it is frequently preferable that the hot zone be generally under a slight positive pressure and because of this no infiltration of air occurs to disturb the desired atmospheric conditions.

Beyond the coolin zone there may be a further extension of the kiln enclosed entirely in metal, as disclosed in Figure 12, wherein the ware is cooled down to normal temperature. In Figure 12 the kiln has a structural metal frame 36 to which is secured a. metal covering 37. This portion of the kiln may have either single or double walls.

While we have shown and described a preferred embodiment of our invention and a method of operating the same, it will be understood that the invention is not limited to the particular construction and arrangement of parts therein disclosed nor to the specific method of operating tem eratures described. The term radiating combustion chamber as hereinafter used shall be understood to be limited to the meaning which the term has acquired in the field of combustion engineer-- ing and which is understood to mean an elongated chamber of nonmetallic refractory having relatively thin walls of a conductivity very much greater than the conductivity of fire clay, silicon carbide generally bein employed, the high conductivity and abillty to radiate heat permitting a high rate of combustion within the chamber with dissipation of the heat therefrom mainly by direct radiation and also by the flow of air thereabout at a rate suflicient to protect the refractory.

We claim:

1. In a tunnel kiln, a kiln structure having a tunnel therethrough, and radiating combustion chambers arranged along the side of the tunnel and extending substantially parallel therewith at a plurality of se arated points in the tunnel, said combustion c amber having the operating characteristics of a combustion chamber of silicon-carbide refrac: tor

2 In a tunnel kiln, a kiln structure having a tunnel therethrough, radiating combustion chambers in the forward part of the tunnel extending substantially parallel therewith, and radiating combustion chambers extending substantially parallel with the kiln intermediate the ends of the kiln and spaced well back of said first combustion chambers.

3. In a tunnel kiln, a kiln structure having a drying zone at the forward end thereof, a preheating zone disposed rearwardly of the drying zone, another heating zone rearwardly of said preheating zone, and having a radiating combustion unit therein, still another zone back of said heating zone, said last zone being longer than the preheating zone and having radiating combustion units therein, and a cooling zone disposed rearwardly of the last zone.

4. In a tunnel kiln, a kiln structure having a tunnel extending therethrough, a radiating combustion chamber disposed alongside the tunnel substantially parallel therewith and arranged to radiate heat into the tunnel, a cooling zone constituting a portion of the kiln,

air circulating passages in the walls of that portion of the kiln constituting the cooling zone, means for circulating air from said pas- 7 sages in the walls of the cooling zone to the radiating combustion chamber.

5. In a tunnel kiln, a kiln structure having a tunnel extending therethrough, a radiating combustion chamber disposed alongside the tunnel substantially parallel therewith and arranged to radiate heat into the tunnel, a cooling zone constituting a portion of the kiln, air circulating passages in the walls of that portion of the kiln constituting the cooling zone, means 'for circulating air from said passages in the walls of the cooling zone to the radiating combustion chamber, and means for controlling the volume of air which is circulated.

6. In a tunnel kiln, a kiln structure, a portion 'of which constitutes a cooling zone, a nonmetallic refractory wall for the cooling zone of the kiln whose thermal conductivity is in excess of .006 cal/cm /sec/C., air circulating passages at the back of said wall.

7. In a tunnel kiln, a kiln structure having a tunnel therethrough, a burner unit located in at least one point along the tunnel, another portion of the kiln constituting a cooling zone and having the inner wall thereof formed of a nonmetallic refractory material Whose thermal conductivity is in excess of i .006 cal/cm /sec/C., air circulating passages back of said wall, and means for conducting air from said passages 'to said combustion unit for supplying preheated air to said combustion unit.

8. In a tunnel kiln, a kiln structure having a tunnel therethrough, combustion units at intervals along the length of the tunnel, a cooling zone at the rear of the kiln, said cooling zone having an inner wall composed of a refractory which has a thermal'conductivity in excess of .006 cal/cm /sec/C., air circulating passages back of said wall in the cooling zone,

-' a passageway with which said air circulating passages connect, said passageway extending along the kiln and having branches leading to the combustion units.

9. In a tunnel kiln, a kiln structure having a tunnel therethrough, combustion units at intervals along the length of the tunnel, a cooling zone at the rear of the kiln, said cooling zone having an inner wall composed of a nonmetallic refractory which has a thermal conductivity in excess of .006 cal /cm /sec/ G., air circulating passages back of said wall in the coolin zone, 'a passageway with which said air circulating passages connect, said passageway extending along the kiln and havmg branches leading to the combustion units, and a damper in each of said branch passages.

10. In a tunnel kiln, a kiln structure having a tunnel therealong, radiating combustion chambers at difierent points along the 11. In a tunnel kiln, a kiln structure hav- 1 ing a cooling zone near one end thereof, said cooling zone having its inner walls composed of a nonmetallic refractory whose thermal conductivity is in excess of .006 cal/cm /sec/C., and means for circulating air behind the inner walls of the cooling zone.

12. In a continuous direct fired tunnel kiln comprising a kiln structure having a ware chamber therealong with a trackway along the bottom of the chamber, cars movable along the trackway, sand troughs along each side of the trackway, sealing plates on the cars extending into the sand troughs, and transversely arranged passageways in the bottom of the kiln structure through which air may circulate to cool the bottoms of the cars.-

13. In atunnel kiln, a kiln structure, a portion of which constitutes a cooling zone, a nonmetallic refractory wall for the cooling zone of the kiln, the thermal conductivity of said wall being in excess of .006 cal/cm /sec/C., and air passages at the back of said wall arranged in definite groups.

14. In a tunnel'kiln, a kiln structure, a portion of which constitutes a cooling zone, a nonmetallic refractory wall for the cooling zone of the kiln, the thermal conductivity of said wall bein in excess of .006 cal/cm /sec/C., groups 0 air passages at the back of said wall, and means for controlling the circulation of air separately in said groups. Y

15. In a tunnel kiln, a kiln structure having a cooling zone, said cooling zone having its inner walls composed of a nonmetallic refractory whose thermal. conductivity IS in excess of .006 cal/cm /sec/C., means for producing air circulation behind the inner walls of the cooling zone, and means for controlling the rate of said air circulation in different parts of the cooling zone.

, In testimony whereof we have hereunto set our hands.

EDWIN B. FORSE. CHARLES F. GEIGER. 

